Combined Addition of Alkenyl and Allenic Anions to Squarate Esters. Direct Competition between Six-Ring and Eight-Ring Electrocyclization of 1,2,4,6,8-Cumulenic Pentaenes

The sequential addition of 1 equiv each of a lithioallene and an alkenyllithium to diisopropyl squarate has been studied. The product compositions resulting from the ensuing reaction cascades were not at all comparable to those resulting from the reverse mode of nucleophile addition. These results require that different intermediates intervene. When the alkenyl anion is the lead reagent, the lithioallene enters predominantly trans and two sequential electrocyclic steps ensue. Initial introduction of methoxyallene causes the second nucleophile to enter preferably from the cis direction, presumably because of oxygen coordination to lithium. The proximal orientation of the unsaturated moieties leads to operation of the dianionic oxy-Cope rearrangement. The second electrocyclic event that transpires following formation of the trans dialkoxide is not of the customary 8pi conrotatory type. Rather, a 6pi disrotatory process occurs exclusively. This "allene effect" is shown to originate from strain, orbital alignment, and conjugative factors operating at the transition state.